High strength is desirable in many paper and paperboard applications. One way to achieve this is by manufacturing dense, high-caliper sheets or boards. This requires the use of large amounts of expensive pulp, and produces a heavy product. Another method of creating high strength in paper products is to add starch as sizing.
In one approach, the sizing process uses cooked starch solutions to impart stiffness or strength to the paper. In the sizing process, the wet web is first dried to a pre-set moisture content and/or is re-wet to achieve uniform moisture content throughout; then the material is fed into a size press where a high loading of gelatinized starch is applied to the paper surface; then the material is dried again. This process yields a strong paper, but involves a number of downstream processes that can be inefficient. Inefficiencies result from the number of steps involved in preparing the substrate, cooking the starch and applying it to form the finished product. A considerable amount of energy is required for these steps, which adds to the costs of the process.
In some instances, gelatinized starch can be added to the wet end of the papermaking process, but its retention on the pulp fibers is often poor. Moreover, the contamination of the hitewater with gelatinized starch leads to increased biological oxygen demand of the effluent, so that the process is environmentally unfavorable.
Ungelatinized starch granules can also be added to the wet end of papermaking, but they are also poorly retained. Such starch granules can gelatinize during the drying process, imparting strength to the paper web once it is dry. Adding starch granules in this manner requires lower amounts of energy to dry the paper web, while also eliminating or reducing the use of a size press. As an alternative, ungelatinized starch granules can be incorporated as fillers. In their native state, ungelatinized starch granules do not absorb water like the gelatinized starches, so they can be applied to paper webs that have not been pre-dried. To apply ungelatinized starch, these granules can be sprayed on the moving moist web, and gelatinization can be effected in the dryer. This yields an improvement in dry strength and stiffness of the paper. However, the spraying process does not disperse starch uniformly throughout the thickness of the paper, leading to anisotropic properties.
There remains a need in the art, therefore, for systems and methods for incorporating and retaining ungelatinized starch fillers in the wet end so that high amounts of these fillers are dispersed uniformly in the paper. These fillers should, desirably, be incorporated so that they are stably anchored to the pulp fibers, allowing them to expand or gelatinize during paper manufacturing without being dislodged. In this manner, the fillers can occupy the interstitial spaces between cellulose fibers more completely, improving the rigidity of the paper product. Furthermore, it is known that high filler content has a detrimental effect on the strength of the wet web before it is dried because the fillers act as spacers and interfere with fiber-fiber bonding. An efficient retention system that attaches the fillers to fibers durably in the wet web can advantageously enhance wet web strength during processing by allowing fiber-fiber bonding to proceed unimpeded.